Transistors used for many liquid crystal display devices or many display devices such as flat panel displays typified by a light-emitting display device utilizing electroluminescence (EL) each include a semiconductor such as amorphous silicon, single crystal silicon, or polycrystalline silicon formed over a glass substrate.
Attention has been directed to a technique in which, instead of the above silicon semiconductor, an oxide exhibiting semiconductor characteristics (hereinafter referred to as an oxide semiconductor) is used for a transistor.
For example, a technique is disclosed in which a transistor is manufactured using an In—Ga—Zn oxide layer as an oxide semiconductor and the transistor is used as a switching element or the like of a pixel of a display device (see Patent Document 1).